Using an experimental radar at the Marshall
Space Flight Centre, scientists are monitoring tiny but hazardous
meteoroids that swarm around our planet.

by Dr Tony Phillips

Our planet is surrounded by a swarm
of alien invaders. They fly through space faster than a speeding
bullet and they're nearly impossible to track with conventional
ground-based radars.

Is Earth in peril? Is it time to head
for the community storm shelter?

Not this time, say scientists. The
interlopers are simply meteoroids, ever-present specks of dust shed
by comets and asteroids. And they pose no danger to Earthlings other
than occasional headaches suffered by satellite operators.

Most meteoroids are tiny - typical
grains span just a few tenths of a millimetre and weigh less than
10-4 grams. Nevertheless, meteoroids pack a considerable
punch as they fly by Earth at speeds ranging from ~10 to 70 km/s
(22,000 to 157,000 mph). A 10-4 gm speck of dust racing
along at 30 km/s carries about the same energy as a very high energy
cosmic ray, ~3 x 1020 electron volts. Collisions with
high-speed space dust can electrify satellites, scrambling software
and triggering mistaken control procedures.

So what can a beleaguered satellite
operator do?

"Not much," says Rob Suggs, the Space
Environments team leader at the Marshall Space Flight Centre Engineering
Directorate. Meteoroids are so numerous they're impossible to avoid
entirely. If ground controllers know when Earth is going to enter
a dense swarm they can orient their craft to minimise cross section
or point sensitive components away from the incoming particles.
"They can also turn off high voltage power supplies, and avoid complex
manoeuvres," added Suggs. "This is what we recommend when we give
satellite operators our meteoroid forecasts."

Recurring annual meteor showers like
the Leonids and Perseids are predictable. But what about times when
Earth passes through an uncharted dust stream? There are plenty
of examples in recent years of meteor
flurries that took sky watchers and satellite operators by surprise.

To provide an early warning system
for such events, Rob Suggs and colleagues Bill Cooke and Jeff Anderson
have built an experimental "forward-scatter radar" at the Marshall
Space Flight Centre in Huntsville, AL. It monitors near-Earth meteoroid
activity around the clock.

"Our system is pretty simple," says
Suggs. "We use a 6-element 'Yagi' antenna and a computer-controlled
shortwave receiver to listen for 67 MHz signals from distant Channel
4 TV stations." The transmitters are over the horizon and normally
out of range. But when a meteor streaks overhead the system records
a brief ping - the echo of a TV signal bouncing off the meteor's
trail.

The radar records about 200 echoes
per hour on an average day, says Suggs. Most are weak reflections
from meteor trails too dim to see with the unaided eye. "We're sensitive
to meteors down to visual magnitude 7.5," noted Bill Cooke. "That
corresponds to meteoroids with masses greater than about 10-5
gm."

Nowadays satellites are well-enough
armoured to withstand a smattering of meteoroid strikes. However,
"mechanical damage isn't the main concern," explains Jeff Anderson.
"The 'plasma effect' is more important."

When a meteoroid hits a satellite,
it can heat the impact site (an area 3 to 5 times larger than the
meteoroid itself) to thousands of degrees Kelvin - rivalling the
surface temperature of the Sun. Usually the entire meteoroid is
vaporised along with a tiny bit of the spacecraft.

"Such an impact forms a plasma cloud
consisting of electrons and ions," added Anderson. "These clouds
start out small, but they can quickly spread a few metres across
the surface of the spacecraft. Electrons, which are light and mobile,
rush away from heavier ions in the expanding cloud - that creates
a charge separation." The electric fields that build up between
positively-charged and negatively-charged areas within the cloud
induce currents in the body of the spacecraft.

"In 1993 a Perseid meteoroid hit a
solar panel on the European Olympus communications satellite," recalled
Cooke. "The project manager [for that satellite] believes the hit
caused a pulse that sent false signals to the control system. The
satellite went into the wrong automated sequencing and, for a while,
they lost control of the spacecraft. It took all the remaining propellant
to regain control, and that was the end of that mission."

Solar sails and sun shields for orbiting
telescopes are also vulnerable to space dust. "The bigger you are
the more likely you are to get popped by something," noted Cooke.
Solar sails have lots of surface area so they make big targets.

In the inner solar system, where the
meteoroid population is greatest, sails will lose one or two percent
of their total area to meteoroid strikes every ten years. "It's
important to build sails out of materials that don't propagate rips,"
says Cooke. "Otherwise meteoroids could pose a big problem for such
spacecraft."

Keeping track of space dust, which
is spread throughout the inner solar system, is a big job. The radar
in Huntsville can only detect meteoroids that crash into Earth's
atmosphere above the southeastern United States.

An interplanetary
meteoroid about 10 microns across.

Suggs noted that a world-wide network
of forward-scatter radars would provide a global picture of the
near-Earth meteoroid swarm. As our planet plows through the dusty
background, scientists could use such a network to sample the meteoroid
population all around Earth's orbit.

The Marshall facility, which Suggs
says is "working beautifully," costs less than 500 US Dollars (plus
a PC to control the receiver and analyse the echoes in real time).
But going global might not be easy. "Our challenge now is to figure
out a calibration scheme that will work everywhere, regardless of
how many transmitters are nearby and how they're distributed. We're
breaking new ground and we still have lots of work to do."

Note: Radar isn't the only
way to detect meteoroids (although it is the best way to record
small ones). In places with dark skies, observers can usually spot
2 or 3 bright sporadic meteors each hour. You can also see the solar
system's swarm of meteoroids glowing in the night sky by means of
reflected sunlight. Sky watchers call such glows the Zodiacal Light
and the "Gegenschein." - Tony Phillips